Doped, pyrogenically prepared oxides

ABSTRACT

Doped, pyrogenically prepared oxides of metals and/or non-metals which are doped with one or more doping components in an amount of 0.00001 to 20 wt. %. The doping component may be a metal and/or non-metal or an oxide and/or a salt of a metal and/or a non-metal. The BET surface area of the doped oxide may be between 5 and 600 m 2 /g. The doped pyrogenically prepared oxides of metals and/or non-metals are prepared by adding an aerosol which contains an aqueous solution of a metal and/or non-metal to the gas mixture during the flame hydrolysis of vaporizable compounds of metals and/or non-metals.

INTRODUCTION AND BACKGROUND

[0001] The present invention relates to doped, pyrogenically preparedoxides, a process for their preparation and their use.

[0002] It is known that pyrogenically prepared oxides can be coated withmetal salts or metal oxides by mixing the pyrogenically prepared oxideswith aqueous solutions of metal salts and then drying and/or calcining.

[0003] Products prepared in this way have the disadvantages a) that thedoping substance is not homogeneously distributed in the entire primaryparticle or b) that, depending on the type of doping, inhomogeneitiesmay occur during mixing. Thus, after doping and calcining, the primaryparticles of the doping substance may separate out and be present withmuch larger diameters than the primary particles of pyrogenic oxides.

[0004] It is therefore an object of the invention to achieve homogeneousdoping of pyrogenically prepared oxides with another substance while atthe same time avoiding problems of the prior art, and more particularly,to avoid the presence of separate primary particles of the dopingsubstance or oxides of the doping substance alongside primary particlesof the pyrogenically prepared oxide.

SUMMARY OF THE INVENTION

[0005] The above as well as other objects are obtained by the presentinvention in the form of doped, pyrogenically prepared oxides of metalsand/or non-metals wherein the basic components are pyrogenicallyprepared oxides of metals and/or non-metals, prepared using flamehydrolysis techniques, which are doped with at least one dopingcomponent at 0.00001 to 20 wt. %, wherein the doping amount maypreferably be in the range 1 to 10,000 ppm, and the doping component isa non-metal and/or a metal or a non-metal salt and/or a metal salt or anoxide of a metal and/or a non-metal, and the BET surface area of thedoped oxides is between 5 and 600 m²/g.

[0006] Another feature of the invention is a process for preparingdoped, pyrogenically prepared oxides of metals and/or non-metals. Incarrying out the process, an aerosol is fed into a flame, such as isused in a known manner to prepare pyrogenic oxides by flame hydrolysisand wherein this aerosol is homogeneously mixed with the gas mixture forflame oxidation or flame hydrolysis prior to reaction. The aerosol/gasmixture is allowed to react in the flame and the resulting dopedpyrogenically prepared oxides are separated from the gas stream in aknown manner. A salt solution or suspension which contains thecomponents of the substance to be doped, which may be a metal salt or anon-metal salt (metalloid salt) or mixtures of both or a suspension ofan insoluble metal compound or non-metal (metalloid) compound, is usedas the starting material for the aerosol. The aerosol is prepared bynebulization using a two-fluid nozzle or using an aerosol generator,preferably by the ultrasonic method.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007] The invention will be further understood with reference to thedrawings, wherein:

[0008]FIG. 1 is a schematic diagram of an apparatus suitable forpracticing the present invention;

[0009]FIG. 2 is an EM photograph of pyrogenic silicon made withoutdoping; and

[0010]FIG. 3 is an EM photograph of pyrogenic silicon made with dopingin accordance with the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

[0011] In carrying out the present invention, aerosol technology is usedto feed into a flame in order to prepare pyrogenic oxides by flamehydrolysis.

[0012] The aerosol may be introduced in a preferred embodiment of theinvention by means of a device such as the one shown in FIG. 1.According to FIG. 1, the main part of the apparatus is a burner 1 ofknown construction, such as is conventionally used for preparingpyrogenic oxides. In this case, the pipes for gas and aerosolintroduction may also be interchanged.

[0013] The burner 1 consists of a central tube 2 which discharges into anozzle 3, out of which the main gas stream flows into the combustionchamber 8 and is there burned off. The inner nozzle is surrounded by thefurther annular nozzle 4 (mantle nozzle), out of which flows mantle- orsecondary-hydrogen to prevent caking.

[0014] According to the invention, a centrally located axial tube 5 islocated inside central tube 2 and terminates a few centimeters upstreamof the nozzle 3 of the central tube 2. The aerosol is fed into the axialtube 5, whereby the aerosol gas stream from the axial tube 5 ishomogeneously mixed with the gas stream from the central tube 2 over thelast section of the central tube 2. The central tube conveys air,hydrogen and, for example, silicon tetrachloride for the pyrolysisreaction.

[0015] The aerosol is produced in an aerosol generator 6 (ultrasonicnebulizer). An aqueous salt solution 9 located in the generator 6contains the metal or non-metal as a salt in dissolved ordispersed/suspended form and is used as the aerosol starting material.The aerosol produced by the aerosol generator 6 is passed through theheating zone 7 by means of a carrier gas stream 10, whereupon the waterevaporates and small, finely distributed salt crystals remain in the gasphase. The flame pipe 11 can be cooled with cooling water 12. Secondaryair can be introduced through part 13 into the chamber 8.

[0016] In a further embodiment of the invention, the aerosol may beintroduced using an annular nozzle, which may be arranged at any angle,preferably at right angles, to the main gas stream.

[0017] The basic components which may be used are the non-metals/metalsaluminum, niobium, titanium, tungsten, zirconium, germanium, boronand/or silicon.

[0018] The doping components which are used may be metals and/ornon-metals and their compounds, provided they are soluble in or can besuspended in a liquid solution. In a preferred embodiment, compounds oftransition metals and/or noble metals may be used for this purpose. Theterm “transition metals” is used herein in its art-recognized meaning.

[0019] By way of example, cerium and potassium salts may be used asdoping components.

[0020] The flame hydrolysis process for preparing pyrogenic oxides isknown from Ullmann's Encyclopedia of Industrial Chemistry, 4th ed., vol.21, page 464, the disclosure of which is relied on and incorporatedherein by reference.

[0021] Due to the fine distribution of the doping component in theaerosol, and also the high temperatures (1,000 to 2,400° C.) duringsubsequent flame hydrolysis, during which the doping components arepossibly further reduced in size and/or melted, the doping medium isfinely distributed in the gas phase during the initial stages ofproduction of the pyrogenic oxide, so that homogeneous incorporation ofthe doping component in the pyrogenically prepared oxide is possible.

[0022] Using the process according to the invention, it is possible todope all known pyrogenically prepared oxides (e.g., SiO₂, TiO₂, Al₂O₃,B₂O₃, ZrO₂, GeO₂, WO₃, Nb₂O₅) with other metal or metal oxides or nonmetal or non metal (metalloid) oxides or mixtures thereof.

[0023] The process according to the invention has several advantages:The aggregate or agglomerate structure of the pyrogenic oxide can beinfluenced by the choice of doping components. Furthermore, the pH ofthe pyrogenic oxide can be affected.

[0024] Catalytically active substances (e.g., cerium or noble metals)which are used as doping components can be distributed almosthomogeneously in the pyrogenically prepared oxide.

[0025] Phase conversion of pyrogenically prepared oxides, for examplefrom rutile to anatase in pyrogenically prepared titanium oxide, can beaffected by doping.

[0026] Using the process according to the invention, combinations ofproperties of pyrogenically prepared oxides which have hitherto not beenavailable, or available only with great difficulty, i.e. for example inprocesses requiring several steps, can be achieved.

[0027] Pyrogenically prepared oxides of metals and/or non-metals, dopedaccording to the invention, can be used as fillers, as supportmaterials, as catalytically active substances, as starting materials forpreparing dispersions, as polishing materials for polishing metal orsilicon wafers in the electrical industry, as ceramic substrates, in theelectronics industry (CMP applications), in the cosmetics industry, asadditives in the silicone and rubber industry, to adjust the rheology ofliquid systems, for heat-resistant stabilization purposes, in thelacquer industry, as a heat insulation material, and the like.

[0028] The invention also provides a device for performing the processaccording to the invention which is characterized in that an additionaltube for introducing the aerosol is arranged, preferably axially, in aburner of the structure known for preparing pyrogenic oxides, whereinthe tube terminates upstream of the burner nozzle.

EXAMPLES

[0029] The burner arrangement used in examples 1 to 4 is shownschematically in FIG. 1.

Example 1 No Doping

[0030] 4.44 kg/h of SiCl₄ are evaporated at about 130° C. and introducedinto the central tube of the burner. 3 Nm³/h of primary hydrogen and 8.0Nm³/h of air are also fed to the central tube. The gas mixture flows outof the inner nozzle of the burner and burns in the combustion chamberand the water-cooled flame tube connected in series therewith. 0.5 Nm³/hof mantle or secondary hydrogen are fed to the mantle nozzle whichsurrounds the central nozzle, in order to prevent caking of the nozzle.An additional 12 Nm³/h of secondary air are fed to the combustionchamber.

[0031] The aerosol flows out of the axial tube into the central tube.The aerosol consists of water vapor which has been produced in an amountof 195 g/h by ultrasonic nebulization of pure distilled water in theaerosol generator.

[0032] The nebulized water vapor is passed through a heated pipe withthe assistance of a carrier gas of about 0.5 Nm³/h of air, wherein theaerosol is converted into gas at a temperature of about 180° C.

[0033] At the mouth of the burner (nozzle 3), the temperature of the gasmixture (SiCl₄/air/hydrogen, water vapor and water aerosol) is 150° C.

[0034] The reaction gases and the resulting pyrogenic silica are passedunder suction through a cooling system, by applying a reduced pressureto the flame tube, and thus cooled to about 100 to 160° C. The solid isseparated from the vent gas stream in a filter or a cyclone.

[0035] The silica is produced as a white, finely divided powder. In afurther step, adhering residues of hydrochloric acid are removed fromthe silica by treating it with water vapor-containing air at elevatedtemperature.

[0036] The BET surface area of the pyrogenic silica is 150 m²/g.

[0037] The production parameters are given in Table 1. Furtheranalytical data relating to the pyrogenic silica obtained are given inTable 2.

Example 2 Doping With Cerium

[0038] The same procedure is used as described in Example 1: 4.44 kg/hof SiCl₄ are evaporated at about 130° C. and introduced into the centraltube of the burner. 3 Nm³/h of primary hydrogen and 8.0 Nm³/h of air arealso supplied to the central tube. The gas mixture flows out of theinner burner nozzle and burns in the combustion chamber and thewater-cooled flame tube connected in series therewith. In the mantlenozzle which surrounds the central nozzle, 0.5 Nm³/h of mantle orsecondary hydrogen are supplied in order to prevent caking. Anadditional 12 Nm³/h of secondary air are supplied to the combustionchamber.

[0039] The aerosol flows out of the axial tube into the central tube.The aerosol is a cerium salt aerosol which is produced in an amount of210 g/h by ultrasonic nebulization of a 5% aqueous cerium(III) chloridesolution in the aerosol generator.

[0040] The cerium salt aerosol is passed through a heated pipe with theassistance of 0.5 Nm³/h of air as carrier gas, wherein the aerosol isconverted into a gas and a salt crystal aerosol at temperatures around180° C.

[0041] At the mouth of the burner, the temperature of the gas mixture(SiCl₄/air/hydrogen, aerosol) is 180° C.

[0042] The reaction gases and the resulting pyrogenically preparedsilica, doped with cerium, are removed under suction via a coolingsystem by applying a reduced pressure and thus cooled to about 100 to160° C. The solid is separated from the gas stream in a filter orcyclone.

[0043] The doped, pyrogenically prepared silica is produced as a white,finely divided powder. In a further step, adhering hydrochloric acidresidues are removed from the silica by treatment with watervapor-containing air at elevated temperatures.

[0044] The BET surface area of the doped, pyrogenically prepared silicais 143 m²/g.

[0045] The production parameters are given in Table 1. Furtheranalytical data for the pyrogenic silica obtained are given in Table 2.

Example 3 No Doping

[0046] 4.44 kg/h of SiCl₄ are evaporated at about 130° C. andtransferred to the central tube in the burner. 3 Nm³/h of primaryhydrogen and 8.7 Nm³/h of air are also fed through the central tube. Thegas mixture flows out of the inner nozzle of the burner and burns in thecombustion chamber and the water-cooled flame tube connected in seriestherewith. In the mantle nozzle which surrounds the central nozzle, 0.5Nm³/h of mantle or secondary hydrogen are supplied in order to preventcaking. An additional 12 Nm³/h of secondary air are supplied to thecombustion chamber.

[0047] The aerosol flows out of the axial tube into the central tube.The aerosol consists of water vapor which is produced in an amount of210 g/h by ultrasonic nebulization of pure distilled water in theaerosol generator.

[0048] The aerosol is passed through a heated pipe with the assistanceof about 0.5 Nm³/h of air as carrier gas, wherein the aerosol isconverted into a gas at temperatures around 180° C.

[0049] At the mouth of the burner, the temperature of the gas mixture(SiCl₄/air/hydrogen, water vapor or water aerosol) is 180° C.

[0050] The reaction gases and the resulting pyrogenic silica are removedunder suction via a cooling system by applying a reduced pressure andthus cooled to about 100 to 160° C. The solid is separated from the gasstream in a filter or cyclone.

[0051] The silica is produced as a white, finely divided powder. In afurther step, adhering hydrochloric acid residues are removed from thesilica by treatment with water vapor-containing air at elevatedtemperature.

[0052] The BET surface area of the pyrogenic silica is 215 m²/g.

[0053] The production parameters are given in Table 1. Furtheranalytical data for the pyrogenic silica obtained are given in Table 2.

Example 4 Doping With Cerium

[0054] The same procedure is used as described in Example 1: 4.44 kg/hof SiCl₄ are evaporated at about 130° C. and introduced into the centraltube of the burner. 3 Nm³/h of primary hydrogen and 8.7 Nm³/h of air arealso supplied to the central tube. The gas mixture flows out of theinner burner nozzle and burns in the combustion chamber and thewater-cooled flame tube connected in series therewith.

[0055] In the mantle nozzle which surrounds the central nozzle, 0.5Nm³/h of mantle or secondary hydrogen are supplied in order to preventcaking.

[0056] An additional 12 Nm³/h of secondary air are supplied to thecombustion chamber.

[0057] The aerosol flows out of the axial tube into the central tube.The aerosol is a cerium salt aerosol which has been produced in anamount of 205 g/h by ultrasonic nebulization of a 5% aqueous cerium(III)chloride solution in the aerosol generator.

[0058] The cerium salt aerosol is passed through a heated pipe with theassistance of 0.5 Nm³/h of air as carrier gas, wherein the aerosol isconverted into a gas and a salt crystal aerosol at temperatures around180° C.

[0059] At the mouth of the burner, the temperature of the gas mixture(SiCl₄/air/hydrogen, aerosol) is 180° C.

[0060] The reaction gases and the resulting pyrogenically preparedsilica, doped with cerium, are removed under suction via a coolingsystem by applying a reduced pressure and thus cooled to about 100 to160° C. The solid is separated from the gas stream in a filter orcyclone.

[0061] The doped, pyrogenic silica is produced as a white, finelydivided powder. In a further step, adhering hydrochloric acid residuesare removed from the pyrogenic silica by treatment with watervapor-containing air at elevated temperatures.

[0062] The BET surface area of the doped, pyrogenic silica is 217 m²/g.

[0063] The production parameters are given in Table 1. Furtheranalytical data for the pyrogenic silica obtained are given in Table 2.

Example 5 Doping With Potassium Salts

[0064] The same procedure is used as described in Example 1, wherein a0.5% aqueous potassium chloride solution is used as salt solution.

[0065] 4.44 kg/h of SiCl₄ are evaporated at about 130° C. and introducedinto the central tube of the burner. 3 Nm³/h of primary hydrogen and 8.7Nm³/h of air are also supplied to the central tube. The gas mixtureflows out of the inner burner nozzle and burns in the combustion chamberand the water-cooled flame tube connected in series therewith.

[0066] In the mantle nozzle which surrounds the central nozzle, 0.5Nm³/h of mantle or secondary hydrogen are supplied in order to preventcaking.

[0067] The aerosol flows out of the axial tube into the central tube.The aerosol is a potassium salt aerosol which has been produced in anamount of 215 g/h by ultrasonic nebulization of a 0.5% aqueous potassiumchloride solution in the aerosol generator.

[0068] The potassium salt aerosol is passed through a heated pipe withthe assistance of 0.5 Nm³/h of air as carrier gas, wherein the aerosolis converted into a gas and a salt crystal aerosol at temperaturesaround 180° C.

[0069] At the mouth of the burner, the temperature of the gas mixture(SiCl₄/air/hydrogen, aerosol) is 180° C.

[0070] The reaction gases and the resulting pyrogenically preparedsilica, doped with potassium, are removed under suction via a coolingsystem by applying a reduced pressure and the particle/gas stream isthus cooled to about 100 to 160° C. The solid is separated from the gasstream in a filter or cyclone.

[0071] The doped, pyrogenically prepared silica is produced as a white,finely divided powder. In a further step, adhering hydrochloric acidresidues are removed from the silica by treatment with watervapor-containing air at elevated temperatures.

[0072] The BET surface area of the doped, pyrogenically prepared silicais 199 m²/g.

[0073] The production parameters are given in Table 1.

[0074] Further analytical data for the pyrogenic silica obtained aregiven in Table 2. TABLE 1 EXPERIMENTAL CONDITIONS DURING THE PREPARATIONOF DOPED PYROGENIC SILICAS Prim. Sec. H₂ N₂ Gas Aerosol Air SiCl₄ airair H₂ core mantle mantle temp. Salt amount aeros. BET No. kg/h Nm³/hNm³/h Nm³/h Nm³/h Nm³/h ° C. soln. kg/h Nm³/h m²/g Doping with ceriumsalt and comparison examples 1 4.44 8.0 12 3 0.5 0.3 150 only 0.195 0.5150 H₂O 2 4.44 8.0 12 3 0.5 0.3 180 5% CeCl₃ 0.210 0.5 143 3 4.44 8.7 123 0.5 0.3 180 only 0.210 0.5 215 H₂O 4 4.44 8.7 12 3 0.5 0.3 180 5%CeCl₃ 0.205 0.5 217 Doping with potassium salt and comparison example 34.44 8.7 12 3 0.5 0.3 180 only 0.210 0.5 215 H₂O 5 4.44 8.7 12 3 0.5 0.3180 0.5% KCl 0.215 0.5 199

[0075] TABLE 2 ANALYTICAL DATA FOR SAMPLES OBTAINED ACCORDING TOEXAMPLES 1 TO 5 Thick. Ce K Cl Grindo Sedi- pH Comp. Lupoda BET wt. wt.conc. LOD LOI Cl meter vol. Effic- 4% b. d. 1 No. [m²/g] μg/g μg/g ppmwt. % wt. % ppm μm vol % iency sus. g/l [mPas] Doping with cerium saltand comparison examples 1 150 — 0.19 1.29 18 0 697 3.98 27 1745 2 1431860 <5 0.09 1.33 20 0 690 3.93 26 1990 3 215 84 <5 45 0.27 1.87 45 1811 422 4.00 25 3390 4 217 2350 <5 112 0.22 2.23 112 40 50 548 3.67 293680 Doping with potassium salt and comparison example 3 215 <5 45 0.271.87 45 18 11 422 4.00 25 3390 5 199 300 55 0.32 1.86 55 60 50 451 4.8332 2575 # Efficiency = turbidity measurement: the method of effciencydetermination (turbidity measurement) is described in Patent DE 44 00170; the suspension prepared by the same method is used for determiningsediment volume after standing for a further 5 minutes; Compacted bulkdensity based on DIN/ISO 787/IX, JIS K 5101/18 (no sieved). Thickeningin polyester reference system: described in EP-A 0 015 315.

[0076]FIG. 2 shows an EM photograph of pyrogenic silica prepared inaccordance with Example 3 (no doping).

[0077]FIG. 3 shows an EM photograph of pyrogenic silica prepared inaccordance with Example 4 (doped with cerium salt).

[0078] It can be seen that the aggregate and agglomerate structure ismodified when doped with cerium salt. Larger cohesive structures areproduced with doping.

[0079] The analytical data for silica in accordance with Example 4, ascompared with that for silica in accordance with Example 3, shows anincreased sediment volume and a greatly increased efficiency value. Thisalso indicates enlargement of the aggregate or agglomerate structure.

[0080] It should be noted that comparable results are obtained inaccordance with the present invention when a noble metal is used inplace of cerium in Example 4. Similarly, when silica is replaced withaluminum oxide, niobium oxide, germanium oxide or boron oxide,comparable results would be obtained.

[0081] Furthermore, using silica doped with cerium in accordance withthe invention, a clear improvement in thickening effect is produced inunsaturated polyester resins.

[0082] Further variations and modifications will be apparent to thoseskilled in the art from the foregoing and are intended to be encompassedby the claims appended hereto.

[0083] German priority application 195 50 500.3 is relied on andincorporated herein by reference.

We claim:
 1. A doped pyrogenically prepared oxide of a metal and/or anon-metal, comprising an oxide of a metal and/or non-metal prepared byflame hydrolysis, which is doped with at least one doping component at0.00001 to 20 wt. % and the doping component is a non-metal and/or metalor a salt or an oxide of a metal or non-metal, and the BET surface areaof the doped oxide is between 5 and 600 m²/g.
 2. The doped pyrogenicallyprepared oxide according to claim 1 wherein the doping amount is in therange 1 to 10,000 ppm.
 3. The doped pyrogenically prepared oxideaccording to claim 1 wherein silica is said oxide.
 4. The dopedpyrogenically prepared oxide according to claim 1 wherein said oxide isan oxide of a member selected from the group consisting of aluminum,niobium, titanium, tungsten, zirconium, germanium, boron, silicon andmixtures thereof.
 5. The doped pyrogenically prepared oxide according toclaim 1 wherein said doping component is cerium.
 6. The dopedpyrogenically prepared oxide according to claim 1 wherein said dopingcomponent is a noble metal.
 7. The doped pyrogenically prepared oxideaccording to claim 1 wherein said doping component is a transitionseries element soluble in water or capable of forming a liquidsuspension.
 8. A process for preparing a pyrogenically prepared oxide ofa metal and/or non-metal comprising feeding an aerosol to a flame forpreparing a pyrogenic oxide by flame hydrolysis, this aerosol beinghomogeneously mixed with a gas stream for flame oxidation or flamehydrolysis prior to reaction, allowing the aerosol/gas mixture to reactin a flame and separating resulting doped pyrogenically prepared oxidefrom the gas stream, wherein a salt solution or suspension whichcontains the component of a substance to be doped, which can be a metalsalt, non-metal salt, or metalloid salt or mixtures or a suspension ofan insoluble metal compound or non-metal or metalloid compound ormixture, is used as a starting material for the aerosol.
 9. The processaccording to claim 8 wherein the aerosol is produced by an aerosolgeneration system.
 10. The process according to claim 8 wherein theaerosol is produced by nebulization.
 11. The process according to claim8 wherein the aerosol is produced by ultrasonic nebulization.
 12. Adevice for preparing a doped pyrogenically produced oxide of a metaland/or a non-metal, comprising a pyrolysis reactor including a heater, asource of aerosol, a conduit to convey said aerosol to said heater, saidconduit being jacketed by a mantel downstream from said heater andprojecting into a combustion chamber, said mantel terminating in anozzle for producing a flame for flame pyrolysis wherein the additionalconduit for introducing the aerosol into the chamber is axially in aburner chamber for preparing said pyrogenic oxide, wherein the conduitterminates upstream of the burner nozzle.